This invention relates generally to generator field windings and, more specifically, to generator field windings configured and disposed on a generator rotor in a manner to reduce hot spot temperatures.
Generator rotors are provided with radial slots about the periphery thereof, for receiving field or rotor windings of coils made up of a number of turns in a radially stacked arrangement, each turn or winding separated by insulation. The windings are retained in the slots by full-length wedges, with creepage blocks interposed between the wedges and the windings.
Generators currently available from the assignee of the present application are placed in three major design classifications based on the cooling medium used: air cooled, hydrogen cooled and liquid cooled. All hydrogen and water cooled generators use direct conductor cooling of the rotor winding for heat removal. Smaller two-pole and all four-pole generators use a radial flow design where hydrogen enters the windings through full length sub-slots and is discharged along the length of the rotor body through radial slots, machined or punched in the copper windings. The hydrogen passes from the conductors through the creepage blocks and wedges to an “air gap” between the rotor and the stator, where it is directed through the stator core to the hydrogen coolers.
At higher generator ratings, and consequently longer rotor body lengths, a gap-pickup diagonal-flow cooling process is employed. In this scheme, cold hydrogen is scooped up in the air gap between the rotor and stator and driven diagonally inwardly through the rotor field turns to directly remove the heat. At the bottom of the slot, hydrogen gas is turned and passes diagonally outwardly through the field turns to the air gap in a discharge stator core section. The stator core ventilation is coordinated with the rotor cooling gas flow, thus creating an in and out flow of hydrogen through the stator core, through the rotor and returning to the hydrogen cooler through the core.
The generator field windings consist of extruded copper that is drawn at a copper mill and then machined and fabricated into a usable coil. Within the last few years, these coils have been redesigned from square corner fabricated coils, to a “C” coil. The cross section of the copper to make these coils has essentially remained the same. The “C” coil has nevertheless been preferred because many benefits have been derived from that shape relating to cost, cycle time and quality. Nevertheless, in order to maintain a competitive stance in the marketplace, new copper designs are constantly being evaluated for increased performance. With the constant design changes, the cross sectional area of the copper has been increasing. When the thickness increases, radial air cooling ducts that are machined in by a punch operation become increasingly more difficult to produce. For example, conventional punch operations may produce an unacceptable bulge in a width dimension of the copper, and it is therefore necessary to create a ventilation scheme for the increasing cross sections of the copper windings without unacceptable bulges in the width dimension.
Furthermore, a design requirement that must be met as part of the industry standards on electrical generators includes meeting specific temperature rise requirements within the capability of certain classes of insulation. One such design requirement is that the maximum field winding temperature stay below a certain limit. In many ventilated field windings, the hot spot temperature is the limiting factor in increasing power density. One method of reducing the hotspot includes increasing the ventilation flow and directing it to regions of high temperature. The disadvantage of increasing flow is that it increases the pumping and windage losses which directly reduce generator efficiency. Another method includes reducing the amount of heat generation in regions of high temperature. For example, one method of reducing the heat generation includes increasing the cross-section area of the field winding copper thereby reducing the resistance and the local heat generation.
The assignee presently makes generator fields with a tapered slot to increase copper content thereby increasing the field thermal capability. In particular, recent generators that use the tapered slot field include tapered slots with either square corner field winding designs or c-coil field winding designs. The tapered slot designs require turns of variable width as a consequence of the slot geometry and may have in the past used thicker turns at the narrow portion of the taper. Furthermore, tapered slots are more costly to machine than parallel slots and turns of variable width in a stack are more complex for manufacturers to make and assemble.
It is desired to reduce local hot spot temperatures on generator fields by reducing heat generation without utilizing variable width turns on a tapered slot design.